1. Field of the Invention
The present invention relates to a method of compensating for the resistance tolerances when printing a multi-tone picture or image with a printing device having printing elements which are arranged in a row. The method is particularly suitable for thermal printing recording devices in which ink particles are transferred onto or into a recording medium from an ink ribbon.
2. Description of the Related Art
In a printing device with multi-tone reproduction, recording dam are generally converted into printing pulses having a pulse width corresponding to the tone grades for supplying to suitable heating resistors. This process controls the proportion of toner transferred from a toner film to give a recording of desired density. The heating resistors are heated to selected temperatures by supplying print pulses based on the tone data. However, when differences exist in the resistance values between the heating resistors, the heating resistors are heated to different temperatures, even if their print pulses are supplied at the same pulse width. As a result, the recorded picture shows irregularities in density and accordingly a poor-quality recording.
However, such differences in the resistance value of the heating resistors are unavoidable side-effects to some extent, since it is difficult to design in a uniform manner the resistance values of the number of heating resistors arranged in a line. Under these conditions, a mean resistance value is given for all heating resistors of the thermal head.
It is further known that heating resistors deteriorate and the resistance values increase over the course of time. If no steps are taken in the thermal head or in the housing of the recording device to compensate for the deterioration of the heating resistors, the thermal head is no longer usable when the resistance value increases by approximately 10%.
A thermal printing recording device with multi-tone reproduction is known from DE 38 20 927. In addition to a thermal head with a number of heating resistors arranged in a row, means are provided for measuring the resistance of each individual heating element and a compensating storage for storing information which is used to compensate for tone data and indicates the tone grades of points or dots to be recorded as a function of the resistance values of the heating resistors for recording the dots. The information stored in the compensation storage varies with the resistance values measured at selected times by the measuring device. The means for driving the heating resistors are suitable for referring to the information stored in the compensation storage.
The compensation storage can be constructed for storing compensation data for determining the pulse width to be supplied to each of the heating resistors from the resistance value of every heating resistor and the tone grades indicated by the tone data. Although this multi-tone recording device is capable of reacting immediately to all changes with respect to the heating resistors of the thermal head, the device is very expensive, and additional time is required between the individual printing processes to detect the resistance. Accordingly the printing process is lengthened, which is undesirable particularly with high resolutions.
The compensation principle implemented in the known multi-tone recording device principally aims at producing every optical density desired on the recording carrier in accordance with the picture data by means of analog-valent ink transfer.
A method of printing a half-tone picture in which a plurality of grade values is generated from a small number of required tone grades by means of a reference pattern allocation or assignment is known from DE 40 25 793.2.